Consistent and reliable tracking of an inserted device is one of the critical requirements for the success of MR-guided (Magnetic Resonance) endovascular interventions and as such, several methods have been reported. These tracking methods may be categorized either as passive, semi-active, or active.
Passive tracking methods exploit the susceptibility difference between the tissue and the inserted device. These passive methods are typically simpler but slower compared to other methods.
Semi-active methods are similar to the passive methods except that the susceptibility difference necessary to cause a signal loss is controlled through external means. This is typically done through the induction of a magnetic field that modifies the MR-signal by activating a coil integrated as part of the inserted device.
The complexity of the inserted device increases substantially with an active method with the addition of an antenna to measure the induced NMR (Nuclear Magnetic Resonance) signals from the surrounding protons to determine the position through three axial projections. Active methods typically provide superior spatial and temporal resolutions. Unlike passive methods, substantial increases in temperature, problematic for human interventions have been recorded in both active and semi-active tracking methods mainly due to the induced current along the length of the conducting wires by the applied RF (Radio Frequency) waves.
Untethered microdevices under development contain a ferromagnetic core and are propelled by magnetic forces induced by the magnetic gradients generated with an MRI system (Magnetic Resonance Imaging). This method is referred to as MRP (Magnetic Resonance Propulsion). In this context, an MRI system is not only used to image the region of interest, but also to propel a ferromagnetic microdevice, determine its location, compute the corrective actions through feedback controls to adjust the generation of the magnetic gradients. These adjustments are necessary to navigate such a microdevice in a pre-planned path inside the blood vessels. As such, being unable to track such a device within specific real-time constraints would prevent the feasibility of such an interventional technique. The main motivation behind MRP is that an untethered implementation may be suitable in order to reduce the risks of encumbrance of the blood vessels and tissue damages in more complex pathways caused by the friction of existing tools such as a catheter or other tethered devices. These microdevices may eventually be useful for performing tasks in remote sites that are presently inaccessible or at high risks with existing tools. These tasks could include but are not limited to thermal treatment of tumors at selected sites, highly localized drug delivery for chemotherapy, on-site delivery of MRI contrast agents, and carriers for biosensing applications.
For human interventions, the overall diameters of such unthetered devices are generally constrained to ˜10.5 mm when operating in the aorta, 1.0-4.0 mm in large arteries, and could theoretically have diameters down to ˜0.006-0.010 mm when operating in capillaries. These values depend on several factors including but not limited to blood flows, the diameter of the blood vessels, the size of the ferromagnetic core inside the device, the shape of the device, the corresponding drag force, the ferromagnetic material being used, and the amplitude and duty cycle of the applied magnetic gradients.
In known passive tracking methods, small paramagnetic rings are typically mounted as markers on catheters and guidewires. These markers produce local field distortions appearing as areas of signal loss in MR imaging in a region surrounding the markers. Furthermore, this positioning by signal loss, referred to as negative contrast, is limited to regions of high signal intensity where the signal loss can be detected without ambiguity. More recently, a novel approach to passive tracking of paramagnetic markers has been described where positive contrast of the markers to their background (white marker tracking) is exploited. With this method, a dephasing gradient is added in the direction of the slice selection during excitation to enhance the contrast between the markers and the background. This compensation gradient induces a signal loss in the image through dephasing and rephrasing of the signal surrounding the area of perturbed magnetic field, resulting in a positive contrast (instead of a negative contrast when no compensation gradient is used), the markers appearing bright on a darker background. A positive contrast can also be obtained with a bead or a coating with is doped with a 4-6% Gd-DTPA solution and applied to the instrument to generate an increase of the MR signal. The Gd-DTPA is characterized by reducing the longitudinal relaxation time of surrounding tissues and shows more signal than biological tissues when the images are acquired with a very short repetition time. The addition of a compensation gradient further improves the contrast between the instrument and the background. Unfortunately, these methods are still image-based methods. Although trade-offs may be achieved between spatial and temporal resolutions, these methods are far too slow in achieving an acceptable spatial resolution to be integrated within the real-time constraints of MRP-based applications.
Recently, a new method for rapid MRI tracking has been described in Kochavi E, Goldsher D, Azhari H. Method for rapid MRI needle tracking. Magn Reson Med 2004; 51:1083-1087, where six central k-space lines were usually sufficient to locate a needle at the cost of an increase in computation. However, this method does not deal with image artifacts and only applies within a given 2D image slice.